In a semiconductor device manufacturing process, a laminated structure of an integrated circuit is formed by repeating a pattern formation and a thin film formation on, e.g., a semiconductor wafer (hereinafter, referred to as a “wafer”) as a substrate. The pattern forming process includes: forming a resist mask by photolithography; forming a pattern corresponding to the mask by performing, e.g., plasma etching, on an underlying thin film by using the mask; and performing ashing for carbonizing the resist mask by an O2 containing plasma.
In the ashing process, residues of the resist mask are generated to remain on the surface of the thin film and/or inside the grooves of the pattern, for example. In order to remove the residues remaining on the surface of the wafer and/or inside the grooves by cleaning, a cleaning fluid is supplied onto the surface of the wafer after the ashing process. Specifically, in a single-wafer spin cleaning device, a cleaning fluid, e.g., pure water, is sprayed from a nozzle to the wafer surface while rotating the wafer. The residues are cleaned by the cleaning fluid while scanning the nozzle from the central portion of the wafer to the peripheral portion thereof. Next, the cleaning fluid remaining on the surface is scattered off by rotating the wafer or evaporated by heating the wafer. Hence, the cleaning fluid remaining on the wafer surface and inside the grooves is removed.
The pattern has various shapes in accordance with portions of a device. Specifically, a wiring pattern formed in an insulating film can be formed as a line-and-space pattern (a pattern in which a plurality of line-shaped protrusions and grooves are arranged in parallel with one another). The line-and-space pattern has an area of a high pattern density and an area of a low pattern density. FIG. 17A shows an example of such pattern. As described above, a surface of a wafer 100 has a high-density area 103 where lines 102 are densely disposed due to, e.g., grooves 101 having a small opening width, and a low-density area 104 where the lines 102 are separated by a the grooves 101 having a larger opening width than that in the high-density area 103. Reference numeral “105” in FIG. 17A indicates residues.
The cleaning fluid remaining on the surface of the wafer 100 after the cleaning process tends to become a substantially horizontal shape so as to reduce its surface area, e.g., in the grooves 101 due to surface tension. Accordingly, a horizontal force is applied to the lines 102 so as to pull the lines 102 toward the cleaning fluid (the grooves 101) due to the surface tension of the cleaning fluid. In the high-density area 103, the surface tension strongly acts due to the small gap between the lines 102 and, thus, it is difficult to remove (dry) the cleaning fluid remaining on the surface of the wafer 100. On the contrary, in the low-density area 104, the surface tension is weaker than that in the high-density area 103 due to the large gap between the lines 102 and, hence, the cleaning fluid in the low-density area 104 is removed (dried) faster than in the high-density area 103. As shown in FIG. 17B, if the cleaning fluid in the high-density area 103 remains while the cleaning fluid in the low-density area 104 is completely removed or remains at a small amount compared to that in the high-density area 103, the attractive force directed toward the high-density area 103 (left side in FIG. 17B) becomes larger than the attractive force directed toward the small-density area 104 (right side in FIG. 17B) in the line 102 located in the boundary between the areas 103 and 104.
Meanwhile, along with the trend toward high integration, there is often the case where the lines 102 have an extremely small width, smaller than about 100 nm, for example. Therefore, the strength of the lines 102 is decreased, and a porous low-k film (e.g., SiCOH film) formed of a porous material and serving as an interlayer insulating film becomes soft. When the attractive forces from both sides are not in equilibrium, the lines 102 collapse toward the side where the larger attractive force is applied (left side in FIG. 17C), as shown in FIG. 17C. In addition, the removal (drying) rate of the cleaning fluid is slightly different between, e.g., the grooves 101 in the areas 103 and 104 as well as between the opposite sides of the line 102 existing in the boundary of the areas 103 and 104. Thus, the lines 102 may collapse after the cleaning fluid is removed (dried) even in the areas 103 and 104 each of which has a uniform arrangement density of the lines 102.
The above resist mask as well as this low-k film may be formed as a mask pattern having a finer dimension compared to the above pattern, wherein, lines may have a width of, e.g., about 32 nm, and a height (depth of grooves) of, e.g., about 120 nm, and the grooves may have an opening dimension of, e.g., about 32 nm. When the mask pattern is formed on the resist mask by a developing process, an organic material forming the resist mask or residues of the organic material may remain on the surface of the mask pattern or inside the grooves of the mask pattern and, hence, the substrate needs to be cleaned after the developing process. Since, however, the resist mask formed of the organic material is of a low hardness, the lines may collapse during the removal (drying) of the cleaning fluid.
The pattern formed by, e.g., etching, may have a shape other than the elongated shape in which the lines 102 are extended along the surface of the substrate. For example, as shown in FIG. 18A, in a double-gate type fully depleted SOI-MOSFET, a columnar structure 110 such as a rectangular pillar for forming a channel on a top surface and a side surface thereof, a gate electrode for forming a double gate structure referred to as a “FIN-FET”, or the like may be formed. Further, a cylindrical electrode 111 may be formed, e.g., on top of the gate electrode, as shown in FIG. 18B. A wafer 100 having such pattern may be subjected to a cleaning process in order to remove residues generated by, e.g., an etching process. When a cleaning fluid is removed (dried) in the cleaning process, protrusions (the structure 110 and the electrode 111) may collapse. If the width of the protrusions of the pattern is reduced along with the trend toward high-density of wiring, the problem of the collapse becomes more serious.
Here, there has been known a method for supplying an organic solvent, e.g., alcohol, which is easily removed (evaporated) due to its low boiling point, onto the surface of the wafer where the cleaning fluid remains, substituting the cleaning fluid remaining on the wafer for the organic solvent, and performing a drying process to remove moisture (the cleaning fluid and the organic solvent) from the surface of the wafer. However, in this method as well, when the organic solvent is removed or dried, the protrusions may collapse due to the surface tension of the organic solvent. Although the surface tension of the cleaning fluid can be reduced by mixing, e.g., a surfactant, with the cleaning fluid, the surfactant remaining on the cleaned wafer surface causes contamination of the wafer.
In addition, although a method for cleaning a wafer by supplying steam onto the surface of the wafer has been known, the steam is insufficient to remove the above-described residues. Besides, there has been known another method for cleaning a wafer by supplying a cleaning fluid in a mist form (droplets) onto the surface of the wafer. However, even if the cleaning fluid is supplied in a mist form, its surface tension still exists. Therefore, if the mist enters the grooves, for example, the horizontal force is applied to the lines. Moreover, if the mist is condensed on the wafer, the lines collapse during the drying process as in the case of the example of FIGS. 17A to 17C.
Furthermore, there has been known a cleaning method using a supercritical fluid obtained by applying an extremely high pressure to, e.g., carbon dioxide (CO2), an organic solvent or the like. However, this method requires a high-pressure equipment and is not easy to be implemented into a device. Although a technique for cleaning a substrate while reducing surface tension of a cleaning fluid by heating the substrate or the cleaning fluid to a temperature within a range from about 50° C. to about 100° C. is described in JP6-196397A (paragraphs 0004 and 0005), such technique can slightly reduce the surface tension but is insufficient to solve the above-described problems.